Download pdf - Compresso CONTROL_TPG Rev1

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Axial

Single Stage

Inter-cooled

Refrigeration

Compressor

Technical Product Guide

Control

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COPYRIGHTby Triconex Systems, Inc.La Marque, Texas, U.S.A.

All rights reserved. No part of this work covered by the copyright hereon may bereproduced or copied in any form or by any means—graphic, electronic, or

mechanical—without first receiving the written permission ofTriconex Systems, Inc., La Marque, Texas, U.S.A.

Printed in U.S.A.1999

NOTE: Triconex Systems, Inc. reserves the right to make improvements in the design, construction,and appearance of its products without prior notice.

Compressor ControlTechnical Product Guide

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Table of ContentsWhat is Surge? ....................................................................................... 7Performance Curves ............................................................................. 7µ Curve .................................................................................................. 7Compressor Map................................................................................... 8A Universal Surge Curve .................................................................. 13Determining HP/A0

2 ..................................................................................................................................13Determining (Q/A0)

2 ..................................................................................................................................14Plotting the Surge Line ...................................................................... 14Summary of Features ......................................................................... 21Choice of Pressure Rise or Pressure Ratio Algorithms ............... 22Safety Margin Recalibration ............................................................ 22Setpoint Hover .................................................................................... 22Dynamic Adaptive TuningTM ......................................................................................................23Non-symmetrical Valve Response .................................................. 23Purge and Start-up Logic................................................................... 24Manual Control ................................................................................... 24Proportional Function ........................................................................ 24Valve Prep ............................................................................................ 25Dump Output ...................................................................................... 25Valve Linearization and Reversal ................................................... 25Compensated Recycle Temperature Controller ........................... 25Process Pressure Control ................................................................... 29

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This document describes theimplementation of a centrifugalcompressor surge control in theTRISENTM digital governor orcontrol system.

What is Surge?

Surge occurs in a turbo compressorwhen discharge head cannot besustained at the available suction flow.Surge occurs at specific combinationsof head and flow, as defined by thecompressor manufacturer's perfor-mance curves. One or more of thefollowing can result from surge:

• Unstable operation• Partial or total flow reversal

through the compressor• Disrupted process• Mechanical damage to the

compressor

Surge is usually accompanied by thefollowing:

• Increase in dischargetemperature

• Reduction in dischargepressure

• Increase in vibration• Sharp rise in inlet tempera-

ture

Performance Curves

A turbo compressor imparts energy tothe gas by accelerating it through arotating impeller to increase itsvelocity and pressure. This kineticenergy is then converted to a higherpressure in a diffuser. The amount ofenergy imparted to the gas is defined

in work done per unit mass of thefluid. Foot pounds per pound (ft-lbf/lb) and kilojoules per kilogram (kj/kg)are the common English and Metricunits. For a given impeller design,diameter, and rotational speed, theamount of work energy expended isdependent on the volume flow at thesuction.

µ Curve

Compressor manufacturers use "µ"curves to define the performance of

individual impeller stages. A typical"µ" curve is shown in Figure 1.

"µ" indicates the polytropic headprovided by the stage for a givenvolume flow at the suction. Each"µ" curve is specific for a given N/

Compressor Surge

A0; that is, for a given compressor

speed, there is only one valid set ofgas properties (MW, Z and k) andsuction temperature (Ts). If the gascomposition and/or suction tempera-ture is variable, multiple curves mustbe provided to show the performanceof the stage. See the previous figure.

The polytropic head developed for agiven fluid (gas) and inlet conditionsis proportional to the pressurecoefficient "µ," as defined by equation(1).

Equation (1)

HU

gp

2

= ⋅µ

where:

Figure 1- Impeller “ µµµµµ”

% Q / N

Pressurecoeff ic ient

µ" "

0 2010 30 40 50 60 70 80

.20

.40

.60

.80

1.0

1.2

1.4

20

40

60

80

100

1

2 3

12

3

32

1

µ

ηpPolytropicEff ic iency

ηp

8

Hp

= Polytropic headµ = Pressure coefficient of

the impeller for thespecific gas

U = Impeller peripheralvelocity ft/sec

g = Gravitational constant32.17 ft/sec2

In the Figure 1 example, the two setsof three curves indicate the perfor-mance of the impeller for threedifferent gas conditions. The term“A

0” is used to define the sonic

velocity of the gas at the suctionconditions and is determined byequation (2).

Equation (2)

A1545 k Z T

MW0s s=

⋅ ⋅ ⋅ ⋅g

where:

A0

= Sonic velocity of thegas at the inlet condi-tions (ft/sec)

k = Ratio of specific heatsCp/Cv

Zs

= Compressibility factorT

s= Inlet temperature °R

g = Gravitational Con-stant, 32.17 ft/sec2

The upper set of curves, in conjunc-tion with equation (1), relate H

p to Q/

N (inlet flow (acfm)/speed (RPM)).The lower curves relate Q/N topolytropic efficiency h

p. Curves 1

through 3 show performance forheavy, medium, and light gases,respectively. The left extreme of eachline indicates the surge point.

Compressor Surge

Compressor Map

While the "µ" curves are a tool usedin the design stage, the final com-pressor characteristic is defined by aperformance curve, or compressormap. This is a series of impeller "µ"curves combined as a multistagecompressor. An example is shown inthe next figure.

NOTE: The Hp versus Q curve is

good for one set of gas conditions(A

0) only.

The Surge Line is not linear withrespect to flow. In a low head, singleimpeller machine, compressing a lightgas such as air, the surge line tends tofollow the fan law. This law states thefollowing:

• Capacity is proportional torotating speed.

• Head is proportional to theinlet flow squared.

• Power required is propor-tional to the speed, cubed.

Multi-impeller high head machinescan have surge lines which have

significant deviations from this idealcurve. As the speed increases, surgemoves to later impeller stages, due tovolume reduction by the initial stages,and consequent lower suction volumeflow available to the later impeller

inlets. The next figure is an example.

Compressor performance maps arepresented in various forms. Mostprovide a basic performance plot, asshown in Figure 3, but with additionalcurves, relating inlet temperature,molecular weight, pressure ratio,suction pressure, and driver horse-

power. Design gas conditions, such ascompressibility factor (Z), and specificheat (k) are usually noted. If Ts and/orMW are variable, several surge curvesmay be plotted, showing surge limitsfor different gas compositions andsuction conditions.

From a surge control standpoint, the

challenge is to keep the compressor

out of surge without wasting energy

on excessive recycling. This requires

that the surge point be precisely

computed from measurable, compres-

Surge Line

ConstantSpeedLines80%

90%

100%

105%HpPolytropic

Head(1000 Ft lbf/ lb)

Q - Inlet Flow (acfm)

0

20

40

60

80

5000 1000 1500 2000 2500

Figure 2Compressor Performance Map

9

Compressor Surge

H pPolytropic

Head(1000 Ft lbf/ lb)

Surge L ine

1 0 0 %Speed

9 0 %Speed

8 0 %Speed

Q - Inlet Flow (acfm)

1 0 5 %Speed

0

20

40

60

80

5000 1000 1500 2000 2500

Figure 3Multi-Impeller, High Head Performance Curve

sor-operating conditions. This goal

will be addressed in the construction

of the Surge Line. First, we will

describe how the TRISENTM system

uses the recycle valve to avoid surge.

10

Compressor Surge

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In the gas compressor section(Figure 4), surge can be avoided byrecycling a controlled portion of thedischarge flow back to the suctionthrough a recycle valve. Recyclingraises the suction pressure andlowers the discharge pressure, whichincreases flow and moves the

Avoiding Surge

operation away from surge. Raisingspeed also moves the compressoraway from surge. This is a tempo-rary solution because it also raisesPd and lowers Ps, which tends todrive the machine back towardssurge.

Suct ion Discharge

Compressor

Valve

Blowof f

Air

Suct ionDischarge

Compressor

RecycleValve

Gas

Figure 4Gas Compressor Anti-Surge Valve

Figure 5Air Compressor Anti-Surge Valve

In the air compressor section (Figure5), a blowoff valve is used to ventthe compressor discharge to atmo-sphere. This does not affect thesuction conditions, but it reducesdischarge pressure and increasesflow, which moves the operatingpoint away from surge.

12

Avoiding Surge

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The problem with both "µ" andperformance curves is that they bothshow the compressor surge character-istics for specific conditions only.Ideally, a curve and a method whichaccurately defines surge for all gas andsuction conditions, is needed.

A Universal Surge Curve

As stated earlier, the surge andperformance characteristics of thecompressor change the function of A

0

or the sonic velocity of the gas at thesuction. An intuitive analysis of the"µ" curve reveals that if Hp/A

02 (head)

is plotted against (Q/A0)2 (capacity), a

curve which defines surge for all gasconditions, suction conditions, andspeeds results. Furthermore, ifperformance lines are plotted as N/A

0,

rather than simply speed alone, theselines are also valid for all conditions.

None of the variables Hp, A0 , or Q

are directly measurable by conven-tional instruments. The challenge isto define and implement this curve interms of variables that we can measurereliably, economically, quickly, andaccurately. This is a formidable task atfirst look.

Determining HP/A02

Let us first examine the Headvariable Hp/A

02:

Equation (3)

HZ T R

MWpa s c= ⋅ ⋅ ⋅ −

⋅1545 1( )σ

σ

where:Hp = Polytropic head in ft-

lbs/lbZa = Average compressibil-

The Surge Line

ity factor (Zs + Zd)/2Ts = Suction temperature

(degrees absolute)MW= Molecular weightRc = Compression ratio

(Pd/Ps)σ = Polytropic exponent

where:

σ = k

k p

−⋅

1

η

where:k = Ratio of specific heats

(Cp/Cv)η

p= Polytropic efficiency

(%)

We have already defined A0:

Equation (4)

A1545 k Z T

MW0

s= ⋅ ⋅ ⋅ ⋅s g

where:A

0= Sonic velocity of the

gas at the inlet condi-tions , ft/sec

k = Ratio of specific heatsCp/Cv

Zs

= Compressibility factorT

s= Inlet temperature °R

g = Gravitational con-stant, 32.17 ft/sec2

Therefore:

Equation (5)

Surge L ine

8 0 %

9 0 %

1 0 0 %

1 0 5 %Hp

A 02

%

Q

A 0

N

A 0

2

( )

Figure 6Universal Surge Curve

14

H

A

Z T R

M Wk Z T

M W

p

a s c

s s02

1

=

⋅ ⋅ −⋅

⋅ ⋅

( )σ

σ

Za/Z

s increases predictably with

increasing head so it can be accommo-dated in the surge line. So by cancella-tion:

Equation (6)

p cH

ARk 0

2

1=

-

σ

σ⋅

And substituting: σ = k

k p

−⋅

1η

Equation (7)

then:H

A

Rk

p c

k

k

p

p

02

1

11

= -

-

−⋅ η

η

An analysis of equation (7) withactual compressor data shows that Hp/A

02 is minimally affected by minor

variations in k and ηp, and is predomi-nantly affected by Rc.

Recall that Rc=Pd/Ps and the chal-lenge has been met. The variables Pdand Ps are readily measured byconventional pressure transducers.

Determining (Q/A0)2

If flow is measured by a conventionalorifice or venturi device, then:

Qh Z T

MW Ps s

s

2 =⋅ ⋅

⋅where:

Q = Flow (acfm)h = Orifice differen-

tialZ

s= Compressibility

The Surge Line

Ts

= Suction temp (°R)MW = Molecular WeightP

s= Suction Press

(psia)

So Q/A0 can be written as:

Q

A

h Z T

MW P

k Z T

MW

s s

s

s s0

2

1545

=

⋅ ⋅⋅

⋅ ⋅ ⋅

Simplifying and canceling:

Q

A

h

P ks0

2

1545

=

⋅ ⋅

Plotting the Surge Line

Because k appears in the denominatorof both terms of H

p/A

02, and Q2/A

02, its

effect is further minimized. If changesin k are ignored, the surge curve canbe plotted as a function of P

d/P

s and h/

Ps. The relationship h/P

s is another

way of saying h compensated forchanges in P

s.

Because Q2 is proportional to h, thenew curve no longer follows the fanlaw. A classic low head applicationwill have a surge line that is closer toa straight line. At high heads, the surgeoften moves to a different impeller.Therefore, high head applications willstill exhibit non-linearity.

A typical surge curve may now looklike Figure 7.

Some applications can use a simplervariation of this surge computationtechnique. If both the head term (P

d/

Ps-1) and the capacity term (h/P

s) are

multiplied by Ps, these terms reduce to

Pd-P

s for the head and simply h for the

capacity.

However, there is a disadvantage ofthe P

d-P

s method, or “Pressure Rise”

method. If the surge line is nonlinearand the suction pressure is variable,then errors will result when operatingon a section of the curve that does notpass through the origin. If the surgeline is linear or if the suction pressureis reasonably constant, this method isperfectly acceptable and can be usedwith confidence.

SURGE LINE

100%

90%

80%

105%

0

25

50

75

100

200 40 60 80 100

h/Ps (%)

% Speed

Pd/Ps

Figure 7Typical Surge Curve

15

The Surge Line

Suct ionDischarge

Compressor

PT

S C

FT

∆p

Recycleor

Blowoff

h

IP

Suct ion Discharge

Compressor

PT

S C IP

FT

Recycleor

Blowoff

PT

Ps Pd

h

Figure 8Pressure Rise Method

Figure 9Pressure Ratio Method

16

The Surge Line

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17

To prevent surge, the system mustaccurately predict surge and begin toopen the recycle valve before surgecan occur. The safety marginbetween the predicted surge point,and the point that recycle flow isinitiated, is called the surge margin.

The surge margin is implemented bymodifying the surge line to producea "Control Line." Referring to

Figure 10, the Control Line lies tothe right of the surge line by anamount equal to the safety margin.Depending on the application, eithera constant (parallel), or progressive(meeting at the origin), characteris-tic can be configured.

For a given Pd/Ps (or Pd-Ps), thesystem computes the surge flowfrom the surge line, then applies the

Figure 10Typical Control Line

Surge Line

0

25

50

75

100

200 40 60 80 100

hc%

ControlLine

Constant

ControlL ine

Progressive

SafetyMargin

SurgeA r e a

SafeA r e a

PdPs

The Surge Control Line

safety margin to determine thecontrol flow.

The control line provides thesetpoint to the surge controller,which opens the recycle valve toprevent flow from falling below theControl Line.

18

The Surge Control Line

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The TRISENTM surge controlsystem can be best described in theform of individual modules. Each

Surge Control Block Diagram

module has a readily definedfunctionality, and interacts withother modules through input and

Figure 11Surge Block Diagram

output signals, which are given tagnames.

In the next section, the features ofthe system will be examined andeach of the blocks will be readilyidentified with an associatedfunction.

M

SPID

D p

1S urgeL ine

C ontro lL ine

C ontro lle rS etpo in t4 5

3R eca lib ra te

hx

6C ontro lle r

hx

2 S urgeD etect

7P roportiona l

Term

12V alveP rep

8

E nable (D I)

D um p (D O )

S tartup

S peedB ias

S peedR eference

10

B ias

(in terna l)

M ARGIN

SUCLN

PROTM

VAPREP

CONSP

DUMP

SULIN

SUVLV

SPDUP

ADM AR

hx

STUP

hx

or

P d

P s

A /M

O pen

C lose

A uto /M an11

13

V alveL inearize /R everse

h hx

VA

F rom A dd ition a l C o m p re sso r S ec tion s(if app licab le)

9

SUCON

T yp ica l fo r E a ch C o m p re sso r S ec tion

H ighS elector

V a lveS igna l

S e lector(if necessary)

R ecyc leV alve(A O )vV 2

20

Surge Control Block Diagram

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21

Summary of Features

Because surge can occur very quickly,special control techniques must beused to ensure that the recycle valveopens in time to prevent surge.

The TRISENTM Controller is ideallysuited to perform surge preventionbecause of its fast processing speedand ability to efficiently perform thecomplex algorithms required. TheTRISENTM controller also performsthe turbine speed control function. Itcan internally implement interactivecoupling between surge and speedcontrol algorithms to improveresponse and stability.

Standard features that can be enabledin the surge prevention strategy are:

• Choice of Pd/Ps vs. h/Ps or∆p vs. h algorithms.

• If a surge occurs, the surgesafety margin automaticallyreadjusts.

• Setpoint Hover functionopens the recycle valve on asudden movement towardsurge.

• Special surge controller withadaptive gain and fastopening/slow closingresponse.

• Proportional function"forces" recycle valve openindependent of controllertuning.

• Speed setpoint is coupled tosurge control.

• Flexible enable logic forstart-up and shutdown.

• Manual control options aidsetup, troubleshooting, andtesting.

• Linearization function forequal percentage valves.

• A Valve Prep function bleedsoverpressure from the

TRISENTM Surge Controller Features

h/Ps %

20 40 60 80 1000

1

2

3

4

5

6

PdPs

x = 60.8y = 5.183

3

x = 92.6y = 5.804

4

x = 40.5y = 3.31

1

2

x = 46.2y = 4.22

recycle valve when operatingclose to the surge line.

• A solenoid contact output“dumps” the recycle valve if

Orifice Differential (h)%

20 40 60 80 1000

0

20

40

60

80

100

∆P

x = 40.5y = 44.9

1

1

x = 60.8y = 86.83

3

x = 92.6y = 98.54

4

2

x = 46.2y = 62.2

2

Figure 13Surge Curve - Pressure Rise

Figure 12Surge Curve - Pressure Ratio

22

surge is imminent, or ifturbine trips.

NOTE: If the application requires it,any of the above features can beenabled. Features not required aresimply not configured and have noeffect.

Choice of Pressure Riseor Pressure RatioAlgorithms

As discussed previously, either of twosurge computing algorithms can beconfigured, depending on the applica-tion.

The surge line can be configured withup to ten line segments. The examplesdepicted in figures 12 and 13 displayfour segments.

Safety MarginRecalibration

If the system detects a transition of theoperating point across the Surge Line,indicating that surge has occurred, itautomatically readjusts the SurgeControl Line to the right to addadditional safety margin.

Some conditions which can result insurge are:

• Shifting of the Surge Linedue to compressor wear

• Transmitter out of calibra-tion

• Insufficient safety margin• Drastic changes in process

conditions• Incorrect surge line used• Improper tuning or set up of

the surge prevention system

Each time a surge transition isdetected, the safety margin isincremented (control line moved to

the right) by a calibrated amount.Entering a new safety margin sets thetransition counter to zero, and sets therecalibrated margin to equal theentered value.

The system can be configured toincrement by a fixed amount (i.e.,2%), or by a progressive amount (1, 2,4, 8%, etc.), on each transition. Themaximum number of times thatrecalibration occurs is alsoconfigurable.

The system displays the following:

• Number of surge occurrences(number of calibrations)

• Initial safety margin• Current recalibrated safety

margin


Setpoint Hover

In most applications, the compressorwill not operate continuously, or forextended periods on the Surge ControlLine. When operation is to the right ofthe control line (safe area), thesetpoint to the Surge Controller isramped (at a configurable rate) towithin a configurable percent of thecurrent h value.

The following occurs after a small,quick movement toward surge, pastthe hover setpoint:

• Immediate opening of therecycle valve

• Hover setpoint is thenramped down (at the samerate) until the recycle valvecloses.

• New operating point isestablished.

If the setpoint reaches the Surge

hx

20 40 60 80 1000

1

2

3

4

5

6

Pd/Ps

Control L ineSurge L ine

Current Operat ing Point hx

rSUCLNrSULIN

rSUCSP

Hover Set t ingk H O V E R

Control ler Setpoint

Hover Ramp Ratek H O V R R

Figure 14Setpoint Hover

h/P S

20 40 60 80 1000

1

2

3

4

5

6

Pd/Ps

Norm alControl Line

Current O perating Point

Hov er Setting

Controller Setpoint

Surge Line

h/P S

Gain Norm al

M in

M ax

Control Line, the system willmaintain recycle flow to operate onthe Surge Control Line.

Dynamic AdaptiveTuningTM

The Surge Controller is available withan adaptive tuning feature that varies

the gain and integral values based onthe margin to the surge line.

Dynamic Adaptive Gain - reducesthe proportional action when opera-tion is to the right of the SurgeControl Line and increases it as theoperation moves to the left of theSurge Control Line. The result is aunique gain value for any operating


point. Normal gain is applied whenthe operating point is on the controlline. As the operating point decreasesto the left of the control line, anonlinear increasing function isapplied to the gain. As the operatingpoint increases to the right of thecontrol line, the gain will decrease toa minimum at the Setpoint Hoverline.

Dynamic Adaptive Integral - allowsa minimum amount of integrationwhen operating on the Surge ControlLine. The Integral value increases non-linearly as the operating point moveaway from the control line (eitherincreasing flow or decreasing flow).This technique allows the surgecontroller to operate near its setpointwith primarily proportional action, yetwhen the operating point moves awayfrom the normal control line, therestoring affect of the integral actionincreases.

Non-symmetrical ValveResponse

The recycle valve opens, based on theproportional and integral responses,but a straight ramp function limits theclosing of the recycle valve. Thisfeature allows the valve to respondquickly to prevent surge. Afteroperation is safely to the right of thesurge control line, the valve is rampedclosed at a slow (configurable) rate toallow the turbine driver and theperformance controller to adjust to thenew operating conditions.

A nonlinear ramp function can alsobe applied that will cause the valve toclose more quickly if the margin ishigh, yet preserves the slow responseas the control line is approached. Thisfeature is useful if the valve movesnearly wide open in response to asevere system transient, thus shorten-ing the time that the operator mustFigure 15 - Dynamic Adaptive Gain

h/P S

20 40 60 80 1000

Pd/Ps

Norm alControl Line

Current O perating Point

Hov er Setting

Controller Setpoint

Surge Line

h/P S

Integral

M in

M ax

wait for the system to return tonormal.

Purge and Start-up Logic

Prior to starting the compressor, therecycle valve can be configured to stayclosed for purging the compressorcase and the inlet and outlet piping.

Once purging is finished, the SurgeControl opens the valve. The SurgeControl can be enabled at anytime, but

is typically configured for one of thefollowing:• Always enabled• Enabled at minimum governor• Enabled by a remote contact or

flag from the DCS

Before the Surge Controller isenabled, the valve is held at the start-up limit. When the Surge Controlleris enabled, the start-up signal isramped closed at a configurable rate


allowing Surge Controller interven-tion as necessary.

Manual Control

Two manual control options areavailable.

The first option provides full-authority manual control. It allows thevalve to be closed, regardless of theaction of the Surge Controller. Thisoption is useful for testing and setup,but should not be configured fornormal operation. If the system is leftin manual operation, the SurgeController will not be able to open thevalve to prevent surge.

The second option provides limited-authority manual control. This optionsets a minimum recycle valve limit,allowing the operator to open therecycle valve, but not close it if thecontroller needs to open it to avoidsurge.

Proportional Function

The system has a proportional-onlyterm, which forces the recycle valve toopen, independent of normal P+Icontroller action. This occurs, due to asevere process upset, if the operatingpoint moves to the left of the SurgeControl Line, and the normal control-ler tuning provides insufficientresponse. This term begins to open thevalve at a specified margin, to the leftof the Surge Control Line, and fullyopens the valve as the operating pointreaches the Surge Line.

In other words, the valve is openedproportional to the instantaneousoperating margin, less the initiationvalue. The proportional term isapplied through a signal selector, andthe anti-windup action of the control-ler forces the controller output totrack the proportional term.

Figure 16 - Dynamic Adaptive Integral

25


This feature will protect the machine,even if the Surge Controller is poorlytuned.

Valve Prep

In some applications, the recycle valveis not equipped with a positioner. Apositioner fully loads the valvediaphragm or cylinder on closed over-travel. This is undesirable because ittakes too much time to bleed off theexcess pressure to open the valve.

The Valve Prep feature is designed toimprove the speed of operation ofthese applications by the following:

• When operating away fromthe Surge Control Line, fullyload or unload the diaphragmto positively close the valve.

• When operation moveswithin a specified distance ofthe Surge Control Line, setthe loading to "poise" thevalve to open.

If the valve is air fail-closed, this

feature requires a special I/P calibra-tion for 6-20 mA input range. The 4-6mA portion is used to under-pressurethe valve closed.

If the valve is air fail-open, thisfeature requires a special I/P calibra-tion for 4-18 mA input current range.The 18-20 mA portion of the range isused to overpressure the valve closed.

Dump Output

When a specified excursion to the leftof the Surge Control Line occurs, acontact output can be configured toopen a high-volume solenoid toquickly open the recycle valve.

This feature is useful on large valveswhich have slower stroking times. Asthe system moves back from surge, thesolenoid will close. This restoresnormal proportional control of thevalve to the Surge Controller.

Valve Linearization andReversal

The TRISENTM Surge Controllerprovides for linearizing the output foran equal percentage trim recyclevalve. This tends to produce a morelinear overall system gain. Possibleinstability is avoided when the systemoperates at a different point than atwhich it was tuned.

Most applications have a recycle valvewith an air-fail open action (AFO),requiring reversal of the recycle valveoutput. The system is configurable foreither direct, or reverse, outputoperation.

Compensated RecycleTemperature Controller

For single component refrigerationcompressors utilizing a liquidquench, Triconex offers a variabletemperature controller setpoint thatchanges with the refrigerant pres-sure. These systems take “hot”discharge gas through a recyclecontrol valve back to the suction andside streams to satisfy the minimumvolume flow demands of the com-pressor sections. A liquid quenchline provides cooling for this gas inorder to maintain the suctiontemperature (and pressure). Figure18 is a schematic of a typical system.

The liquid quench flows are regu-lated by a temperature controllerreferencing the compressor suction orsidestream temperature. As long asthe gas pressure remains within anarrow range, the controller willprovide adequate temperatureregulation.

When the gas pressure rises to thepoint that the temperature controllersetpoint is less than the saturation

100%

0 %Closed

Open

rSULIN

hx%

Py%

0

7

rPROTM

rSUCLN

rMAR

Proport ionalInitiation

Figure 17Proportional Function

26


PT

TT

PT

TT

PT

TT

PT

TT

FTFT

FT

RecycleValve

LiquidQuench

Valve

temperature (at that pressure), thecontroller will open the quench valvefully, even though that amount ofliquid is not required for controllingthe gas temperature. This situationcan result in excessive liquid refriger-ant consumption, high suction drumliquid levels and liquid carry-over intothe compressor.

Figure 19 shows the a typicalpressure-enthalpy diagram for asingle component refrigerant. PointA is the intersection of the gaspressure (P1) and the temperaturecontroller setpoint temperature line.If the pressure rises to P2, theintersection of P2 and the tempera-ture controller

Figure 18Typical Refrigeration System

setpoint is Point B, which is in thesubcooled liquid section of thediagram. Since this temperaturecannot be achieved, the controllerwill open the valve fully.

Also shown on Figure 19 is a line inthe superheat region that parallels thesaturated vapor line. This line

27

Figure 19P-h Diagram for Refrigerant

Enthalpy

Pre

ssur

e

Temperature

Entropy

A

B

P 1

P 2

Convent ionalTemperature

Control lerSetpoint (f ixed)

CompensatedTemperature

Control ler Setpoint

represents the Compensated RecycleTemperature Controller setpoint.Instead of using a fixed temperaturefor the controller setpoint, theequations of state of the refrigerantare used to calculate the saturationtemperature at the gas pressure. Thesetpoint of the controller is thesaturation temperature plus anincremental amount of superheat.


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29

Dynamic Process Control Decoupling

Process Pressure Control

In variable speed applications,compressor capacity is controlled byvarying the speed to meet processrequirements of flow, suction pres-sure, suction temperature, dischargepressure, etc. This controller can beinternal to the system, or it can beexternal and applied as a remote input.

For a given head, compressor speedproduces flow, as defined by themanufacturer's performance curves.Because both the capacity controller(sometimes called the processcontroller or performance controller),and the surge controller influenceflow, there is interaction betweenthem. Depending on the type ofcapacity control, this interaction cancause system instability and makesurge protection extremely difficult.

Consider the following situation:

The system is configured as asuction pressure controllercascading to the speed controller.If the suction pressure falls, thecontroller slows the turbine downto maintain the suction pressure.Conversely, it raises the speed ifthe pressure rises.

An upstream process upsetdrastically reduces the availablegas at the suction. Because thesuction pressure is falling (pres-sure ratio rising) and the flow isfalling, the operating point movesrapidly towards the surge line.

The Surge Controller beginsopening the recycle valve. How-ever, due to valve stroking and the

volume of piping, intercoolers,etc., there can be several secondsdelay before the recycle flowreaches the suction of the com-pressor.

Meanwhile, the suction pressurecontroller sees only suctionpressure falling and responds byslowing down the machine. Thisreduction in speed further reducesthe flow and drives the compres-sor even faster into surge.

If the process controller was control-ling discharge pressure, then thescenario would have been different.The discharge pressure would havedropped and the controller would haveraised speed to correct it. This wouldbe a desirable response, as it wouldmove the operation away from surge.

In some applications, the cycledescribed in the suction pressurecontrol example above can continueindefinitely. To prevent this occur-rence, the Triconex capacity controlleris designed to open the recycle valveonce operation is decreased to thepoint where thecompressor is onthe surge controlline. A separate setof tuning con-stants for theprocess controllerare provideddepending whetherspeed is beingcontrolled or therecycle valve isbeing opened.

Compressorcapacity control is

0 100%

0

100%

0

100%

50%

Load Sharing Control lerOutput

Sta

tion

Rec

ycle

Val

ve O

utpu

t

Tur

bine

Spe

edC

ontr

olle

r O

utpu

t

Turbine Min.Speed

achieved by adjusting the speed of thecompressor and opening the recyclevalve. Maximum capacity is reachedwhen the turbine reaches its maximumoperating speed and the recycle valveis fully closed. Minimum capacity isreached when the compressor isoperating on the recycle line and therecycle valve is fully open.

In effect, the Triconex DynamicProcess Control Algorithm is a specialsplit-range controller with a variablebreakpoint. The point at which thecapacity control switches fromcontrolling the speed to controllingthe recycle valve opening is called theDynamic Breakpoint. This point isdynamic because it is variant depend-ing on the process flow and pressuredemands.

A conventional split range controlleris shown in Figure 20. This type ofcontroller does not take into accountthe slope of the surge control line. Ineffect, the split line must be based onthe highest operating pressure ratio.The turbine minimum speed is based

Figure 20 - Conventional Split RangeProcess Control

30

Flow2

X-50 100%

0

100%

0

100%

50%

Rec

ycle

Val

ve O

utpu

t

Tur

bine

Spe

edC

ontr

olle

r O

utpu

t

Turbine Min.Speed

Minimum BreakPoint (Xm)

Dynamic Break Point (X')

P 2/P1

Process Control lerOutput

Figure 21 - Process Decoupling using Dynamic Break Point

on the maximum operating pressureratio rather than the minimumgovernor speed.

Triconex incorporates the slope of thecompressor curves into the determina-tion of the break point for the splitrange controller. The lower thepressure ratio, the greater the turn-down available to the speed-controlportion of the controller. Figure 21shows how the compressor surge linerelates to the determination of theDynamic Break Point TM. As thecompressor pressure ratio increases,the minimum speed at which surge

occurs also increases. The breakpoint for the split range controllerwill shift to the right to assure thatthe recycle valve is opened based onthe process flow demand rather thanthe surge controller requirements.

The decoupling exit line will be set2% to the right of the surge controlline. In other words, the processcontroller decoupling algorithm willoperate the recycle valve whenever theprocess controller output drops belowthe surge controller control line flow,but does not start closing the valveuntil the process controller output is

2% greater than the surge controllercontrol line. As the operating pointincreases above the surge control line,the process controller will firstincrease speed until the its output is2% greater than the surge control line;further increases in the processcontroller output will cause therecycle valve to close. This approachassures that the valve will not close atthe same time that an increase in thepressure ratio is taking place.

Another feature of the capacitycontroller is that if load is quicklyreduced, the speed setpoint of the


31

Figure 22 - Process Controller Decoupling Block Diagram

turbine is initially reduced a smallamount and then ramped down slowly,as required to satisfy the capacitycontroller. While the speed setpoint isramping down, if further reduction incapacity is required, the recycle valveis opened. This provides superiorcontrol to the traditional method ofreducing speed and finding too latethat operation has crossed to the leftof the surge control line, requiring therecycle valve to open quickly. Anotheradvantage is that since the recyclevalve is opened sooner and closes

gradually, the load on the compressoris maintained which aids in maintain-ing process stability. Note that this isonly a transitory situation; at steadystate the recycle valve is only openedthe amount that is required to main-tain the recycle line.


PT

PT

FT

P d

P s

h

M

SPID

SurgeController

ProportionalTerm

Startup

ST U P

>

M

SPID

SpeedController

G ov ernorValv e

R ecycleValv e

D ecoupling

Surge L ine

M

SPID Process

Controller

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33

Nomenclature

ACFM Actual Cubic Feet Per Minute at flowing conditions C Constant Cp Specific heat of gas at constant pressure Cv Specific heat of gas at constant volume FE Primary flow element, orifice, flow tube, etc. G Gas specific gravity = MW/28.966 h Differential head produced by flow element, usually “H

20

differential hc h corrected for suction pressure = h/Ps Ha Head, Adiabatic, Foot-lbs/Pound Hp Head, Polytropic, Foot-lbs/Pound k Ratio of specific heats, Cp/Cv M Mass flow rate, e.g., Lbs/Minute, Kg/Hr. MW Molecular Weight, Lbs/mole volume N Rotative Speed, Revolutions Per Minute, RPM P Pressure, Pounds per square inch absolute, psia Pb Pressure at base conditions, usually 14.7 psia Pd Pressure (absolute) compressor discharge Pf Pressure (psia) at flowing conditions used to

specify/calculate primary flow element Ps Pressure (absolute) at compressor suction Q Gas volumetric flow rate at flowing conditions (e.g.,

ACFM,SCFM, m3/h, etc.)

R Gas Constant = 1545.3/MW Rc Compression Ratio, Pd/Ps T Temperature, °R = °F + 459.67 or °K = °C + 273.15 Tb Temperature at Base Conditions, usually 60°F Td Gas Temperature (°R or °K) at compressor discharge Tf Temperature (°R or °K) at flowing conditions used to

specify/calculate primary flow element Ts Gas Temperature (°R or °K) at compressor suction Z Compressibility factor to correct for the deviation from the

ideal gas flow. ρ Density, lbs/Ft3

ρ = ⋅⋅ ⋅

P MW

T Z10 73.

34

SYMBOL C ρ P T ZVARIABLE CONSTANT DENSITY PRESSURE TEMPERATURE COMPRESSIBILITY

FACTORSYSTEM UNITS

ENGLISH 10.73125 Lbs/Ft3 psia °R = °F + 459.67 DimensionlessMETRIC 0.084784 kg/M3 kg/cm2

°K = °C + 273.15 DimensionlessS.I. 8.3145 kg/M3 kPa °K = °C + 273.15 Dimensionless

σ Exponent of Compressions, adiabatic or polytropic

Adiabatic ( )

σ =−k

k

1

Polytropic ( )

ση

=−⋅

k

k p

1 where ηp = Polytropic efficiency

Nomenclature

35

Technical Product Guide Notes

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36